Pests, from microbial pests to insect pests, destroy inordinate amounts crops. Improved methods for protecting plants from pests are therefore desired since they would increase the amount and stability of food production. However, pesticides can be difficult to apply, and expensive to maintain after application. Applying toxicants as dry pesticides called dusts is not desirable because it leads to uncontrolled drifting of potentially dangerous chemicals. Applying toxicants in liquids as pesticidal sprays leads to less drifting than dust applications. Nevertheless, regardless of the formulation and application method, the efficacy of a toxicant primarily depends upon its delivery to the target organism.
Pesticidal sprays typically leave residues on plant surfaces. These residues represent an inefficient manner for contact with insects and other pests. Poor pesticide delivery leads to sub-lethal doses of pesticides. Often times, even with adequate spraying or delivery, residues from conventional pesticidal sprays delivered by a water carrier alone do not provide a proper (e.g., lethal) dose to an insect. Efficacy of a pesticidal residue on a plant surface requires that the residue remain on the surface long enough to contact a pest. Surfaces such as leaves, bark, soil, and wood may undesirably absorb pesticide residues and therefore lower the effectiveness thereof. In other words, pesticides loose their effectiveness when applied to sorptive surfaces since contact with pests is inhibited.
Moreover, pesticide application can result in reduced photosynthesis. Generally speaking, pesticide sprays cause a short-term and long-term reduction in the rate of CO2 uptake (necessary for photosynthesis) and enhance leaf senescence. Thus, although plant survival may increase with a pesticide application, decreased transpiration and decreased photosynthesis undesirably occurs. Photosynthesis and transpiration in plants are positively linked in that a decrease in transpiration generally leads to a decrease in photosynthesis.
In perennial crop production such as tree fruit, flower buds for the subsequent year are initiated while fruit are developing for the current growing season. In practice, a plant may or may not produce flower buds for the subsequent year. One of the many biochemical cues to develop flower buds is the rate of photosynthesis and the availability of photosynthetically derived carbohydrates for flower bud development.
The availability of carbohydrates is limited by the photosynthetic capacity of the plant and the pool of carbohydrates is partitioned between the competing carbohydrate needs of the woody tissue, leaf tissue, developing flower buds and developing fruit. If photosynthesis is limited during the flower bud initiation period, flower bud initiation is reduced and fewer flowers are produced the following season. Reduced flower number results in reduced fruit number. In the subsequent year, the tree has a reduced number of fruit and it develops excessive numbers of flower buds because it lacks the competing developing fruit when flower buds are initiated. The alternating production of large and small numbers of fruit is an undesirable condition known as “alternate bearing”.
A related problem to alternate bearing is called “excessive fruit drop”. Normal fruit drop occurs when, simultaneously, the fruit is developing, tree growth is occurring, and flower buds are being initiated. Photosynthetically derived carbohydrates become limiting to all the growing tissues at this time in the growing season and the plant aborts the developing fruit, and limits the initiation of flower buds. When pesticide application deleteriously effects or diminishes photosynthesis, fruit drop is excessive.
Particle carriers for pesticides are generally suitable for control of soil-borne pests. They are not frequently used for foliar control of insects on plants due to difficulties associated with sticking to the foliage, impeding photosynthesis, and/or consequent susceptibility to removal by wind, rain, or other disturbing forces. Particle carriers for plant protection are not necessarily efficient or economic in view of these difficulties.